Add a German learning flow

[paolorechia]

This is an use case for my own personal use :)

I'd like to have a flow / prompt that detects user mistakes and offers corrections/suggestions, all in a specific language. Currently, I'm interested in trying this with German - though in theory it could also work for other languages.

Useful fine-tuned model for German: https://github.com/avocardio/Zicklein